1. Field of the Invention
This invention is directed to self contained backpurge systems for material processing mills as wells methods for using such backpurge systems. In addition, this invention encompasses self contained backpurge systems useful for totally enclosed processing mills and for clean in place (CIP) processing material processing mills.
2. Description of the Art
When processing powders it is desirable to completely contain the powders inside the process housing. Therefore it is beneficial to create a process housing which is totally enclosed and impervious to air and the powder which is being processed.
In the case of a milling device with a rotating impeller assembly, the totally enclosed process housing includes the container which holds the feed material, feed device, mill chamber, including the rotary rotating impeller assembly, and the container which holds the material which is discharged from the mill chamber.
There are several problems with creating totally enclosed process housing. The first problem is that it is difficult to seal all of the various mechanical connections, which are assembled in order to create the totally enclosed process housing, and therefore the housing is subject to leaking. The second problem is that the rotating impeller assembly acts as a fan and tends to blow the product and any gas in the process area down into the discharge container. If the process does not include a way to vent gas from the discharge containers, the gas pressure tends to build up in the discharge container. This can cause material to be held up in the milling chamber. Material held up in the milling chamber is subject to the rotating impeller for longer periods of time and the particle distribution created in the milling may change from the desired particle distribution. It is also possible that material held up in the milling chamber will cause blinding of the classifying screen and cause the further process problem effectively stopping the milling process. A positive pressure in the milling chamber increases the possibility that dust can escape through a connection that is not perfectly sealed.
A further desired (optional) feature of a milling process is to operate in an inert atmosphere. This is typically done by injecting nitrogen gas along with the feed material. Therefore this nitrogen gas and any existing gas in the process area must be vented from the process area.
In order to avoid the above stated problems, current systems vent process gas and nitrogen from the discharge of the mill. The current system uses a set of filter bags in discharge housing. A vacuum system is used to draw out the process gas through the filter bags. The objective of the vacuum system is to create a slight vacuum in the discharge of the milling chamber, which has the desired effect of keeping the product in the mill chamber. The filter bags can be blinded by the dust and lose their effectiveness to draw air, nitrogen, or process gas through the filters. When the filters because blinded, existing systems use a backpurge to alternatively clean the filters. The back-purging uses compressed air or compressed nitrogen and subsequently adds additional process gas to the milling system.
This additional process gas must be removed and often the vacuum system must be increased to remove the additional gas used to clean the filters. Increasing the vacuum causes more dust to adhere to the filters and requires more frequent or stronger backpurge with the additional gas. This system can become unstable and reach a point where a vacuum cannot be held in the milling enclosure and this makes it subject to powders leaking to the outside (particularly hazardous powder).